SM0486
d. The hydrostatic-head gage employs one or more elastic chambers and
differs from ordinary types only in the graduation of the scale.
The scale is
usually graduated to show the head of water, or other liquid, in feet. It may also
read in "psi" pressure.
The gage finds uses in hot water heating systems, for
example, to show the head of liquid in the system.
e. Other gage names which are self-descriptive of their service include
"boiler", "welding", and "sprinkler".
Generally, the gages have been specially
designed, or are modifications of a standard type, to meet specific codes or
specifications.
(7) Calibration of Gages.
In calibrating pressure or vacuum gages, a
pressure standard is used to produce a known pressure or vacuum.
With a known
pressure or vacuum the error (if any) of the gage being calibrated is determined.
In this discussion we will assume that we have a standard pressure available and we
will therefore concern ourselves only with the mechanics of gage calibration.
a. One of the first things to remember when preparing to calibrate a
pressure gage is that the gage must be mounted in exactly the same position as in
its normal installation.
For example, if the gage is normally installed on the
horizontal position, it should be calibrated in the horizontal position.
b. The deviation in indication between the calibration standard and the
gage under test fall into one or more of the following classifications:
Friction.
Range error.
Scale shape error.
Zero shift.
c. Let's look at each of these types of errors and discuss their nature
as well as their remedy.
(1) Friction.
a. Friction is the difference between the pointer indication
before and after light tapping.
Additionally, this condition may be observed by
sluggish or erratic pointer movement either up or down scale.
Friction may be
caused by many factors, the most common of which are dirt and excessive wear.
b. The
correction for friction involves cleaning all bearing
surfaces and gear teeth.
This -an usually be done by gentle agitation in a bath of
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